Add JS methods to get current `center` and `zoom` values

[lindseydiloreto]

If a front-end map has been moved or zoomed by the user, we may need to know the current center coordinates and zoom level. It's already possible to get this information, but it's a little ugly...

let map = googleMaps.getMap('MAPID');
let center = map._map.center;
let zoom   = map._map.zoom;

We should add a couple of methods to make this a little more intuitive...

let map = googleMaps.getMap('MAPID');
let center = map.getCenter();
let zoom   = map.getZoom();

[lindseydiloreto]

Similarly, we should elevate getBounds...

// Wrapper for `map._map.getBounds()`
let bounds = map.getBounds();

[lindseydiloreto]

This is done, and will be available in v4.1.4